The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
As explained in the “Special Report on Water” in the 20 May 2010 issue of The Economist, in this century, a shortage of fresh water may surpass a shortage of energy; and these two challenges are inexorably linked. Despite an increased focus on this issue, many subsequent reports, including the World Health Organization's “2014 Update on the Progress of Drinking Water and Sanitation”, describe the uneven and unequal progress being made towards achieving fresh water supply goals.
Current estimates indicate that approximately 748 million people still lack access to an improved drinking water source. Of these, almost a quarter (173 million) rely on untreated surface water. Markedly, the discrepencies in accessing improved sources of drinking water correspond directly with geographic, sociocultural and economic inequalities.
The hazards posed by an insuficiency and/or incontinuity in the supply of clean water are particularly acute. Continuity of a water supply is taken for granted in most industrialized countries, but is of great concern in many developing countries. Recent estimates indicate that approximately half of the population of developing countries receive water on an intermittent basis, such as water being provided for a few hours per day, or a few days per week. Further still, the supply of fresh water is often seasonally inconsistent. Global warming, and any climate changes brought about, may further threaten these regions.
Understandably, an insufficient and/or intermittent supply of fresh water may lead to a variety of crises, including famine, disease, death, forced mass migration, cross-region conflict/war, and collapsed ecosystems. Despite the crucial need for fresh water, supplies of this liquid are nonetheless constrained.
Although nearly 70% of the Earth's surface is covered by water, 97.5% is saline and occan-bascd. Of the remaining 2.5% (fresh) water, only 1% is easily accessible, i.e. found in underground aquifers. Adding to global shortages of fresh water is the fact that the distribution of freshwater that is easily available is vastly unequal. According to the World Business Council for Sustainable Development, more than half of the naturally-occuring fresh water supply is contained in regional drainage basins located in just nine countries (United States of America, Canada, Colombia, Brazil, Democratic Republic of Congo, Russia, India, China and Indonesia).
As the world's population escalates, a consequential shift in farming and industrialization occurs in order to support this growing number of people. These shifts further intensify the demands for clean, fresh water. Urban areas have a greater need for water beyond the basics for drinking and sanitation. As naturally occurring fresh water is typically confined to regional drainage basins, transport of fresh water to urban localities must be undertaken with ever-increasing costs.
Summarily, these increases in urbanization and population, as well as global warming, all have an impact on the environment, and contribute to the ecological consequences of drying reservoirs and falling aquifers. The resultant fresh water shortages necessitate methods of obtaining fresh water from non-fresh water sources such as, but not limited to, seawater, brackish water and/or even waste water.
In light of this issue, a number of apparatus have been developed to overcome fresh water shortages. These devices operate by separating pure water from a feed liquid selected from the group consisting of, but not limited to, seawater, brackish water and/or waste water. The term waste water includes flowback water and water produced during an oil or gas extraction/production process.
Flowback water or “backflow” water is further defined as a fracturing fluid mixture obtained after an extraction process (recovered fracturing fluid and produced water). It is also known as the recovered water and fracturing fluid which flows back from an oil or gas well drilling fracturing process. Both the fracturing chemicals and fresh water that is injected into the well during the fracturing process tend to dissolve salts in the rock formation, thus increasing the salinity of the flowback water. As such, flowback waters are typically high in salinity and total dissolved solids (TDS), and comprise from about 10-60% percent of the volume of fluid that was initially injected into the well. Flowback waters can also contain contaminants that are present in a rock formation undergoing a fracturing process.
Although there are many existing processes for producing fresh water from seawater brackish water, and/or waste water, the majority of them require massive amounts of energy. For example, despite being the current leading desalination technology, reverse osmosis (RO) is energy intensive and still relatively inefficient due to the high pressure required to drive water through membranes and their tendency to foul. In large-scale plants, the specific electricity required can be as low as 4 kWh/m3 at 30% recovery, as compared to the theoretical minimum of around 1 kWh/m3; while smaller-scale RO systems (e.g., aboard ships) are even less efficient.
Other existing seawater desalination systems include a thermal-energy-based multi-stage flash (MSF) distillation process, and a multi-effect distillation (MED) process; both of which are energy- and capital-intensive. Furthermore, in the MSF and MED systems, the maximum brine temperature and the maximum temperature of the heat input are limited so as to avoid calcium sulphate precipitation, which leads to a formation of a hard scale on the heat transfer equipment. As such, when these technologies are employed, they are usually done on a large scale and are mainly suitable for those economically advanced and resource-rich regions of the world, as many developing countries lack sufficient energy resources to carry out these methods.
One solution is to develop small-scale desalination technologies which can utilize solar energy. Humidification-dehumidification (HDH) desalination systems, are considered advantageous alternatives in providing low to medium scale water production for remote and off-grid areas, and thus provide a promising technology to resolve the issue of fresh water scarcity.
Key components of the HDH desalination systems include a humidifier, a dehumidifier, a compressor and an expander. They are also considered to be the same key components in a basic varied-pressure HDH desalination cycle; one of the many configurations of HDH desalination cycles. Of these components, the heat and mass transfer devices (humidifier and dehumidifier) play a major role in the HDH systems, and use a carrier gas, such as air, to communicate energy between a gas and a liquid, such as a seawater brine. In the humidifier, hot seawater comes into direct contact with dry air, and this air becomes heated and humidified. In the dehumidifier, the heated and humidified air is brought into (indirect) contact with cold seawater and gets dehumidified, producing pure water and dehumidified air.
In light of fresh water demands, efficient, high-performance humidification dehumidification desalination (HDH) systems are necessitated. Those systems comprising a low-cost humidifier possessing an efficiency rating of 85±5% are greatly preferred, as the overall efficiency of the HDH system will also be directly increased.
There are several devices that could be used as a humidifier for the HDH systems. These devices include, but are not limited to, packed bed towers, spray towers, wetted-wall towers, and bubble columns. [R. E. Treybal Mass transfer operations. 3rd edition New York: McGraw-Hill 1980 Incorporated herein by reference in its entirety.]
In a spray tower, water is sprayed at the top of a cylindrical column and falls in the form of droplets —due to gravity—while a running air stream flows upward, so as to be in direct contact with the water droplets. Mist eliminators are essential in this situation, so as to avoid water entrainment in the air leaving the column. These types of humidification devices have a low effectiveness due to their low water hold up. Moreover, the pressure drop in the water stream is high due to the losses in the spray nozzels. [F. Kreith and R. F. Bohem; Direct-contact heat transfer. Washington: Hemisphere Pub. Corp., 1988 Incorporated herein by reference in its entirety.]
As for wetted-wall towers, these devices suffer from a low water flow rate capacity since the water only flows on the inner surface of the tower, and they are not preferred. Conversely, the packed bed tower is widely used as a humidifier in HDH systems [Said Al-Hallaj, Mohammed Mehdi Farid, and Abdul Rahman Tamimi. “Solar desalination with a humidification-dehumidification cycle: performance of the unit”. Desalination 120.3 (1998), pp. 273-280. Y. J. Dai, R. Z. Wang, and H. F. Zhang. “Parametric analysis to improve the performance of a solar desalination unit with humidification and dehumidification”. Desalination 142.2 (2002), pp. 107-118.93. A. S. Nafey, H. E. S. Fath, S. O. El-Helaby, and A. Soliman. “Solar desalination using humidification-dehumidification processes. Part II. An experimental investigation”. Energy Conversion and Management 45.78 (2004), pp. 1263-1277. Ghazi Al-Enezi, Hisham Ettouney, and Nagla Fawzy. “Low temperature humidification dehumidification desalination process”. Energy Conversion and Management 47.4 (2006), pp. 470-484. G. Prakash Narayan, Maximus G. St. John, Syed M. Zubair, and John H. Lienhard V: “Thermal design of the humidification dehumidification desalination system: An experimental investigation”. International Journal of Heat and Mass Transfer 58.12 (2013), pp. 740-748.11-15. Incorporated herein by reference in their entirety.] However, in some operations, the fluid passing through the packed bed may contain suspended solid particles that can accumulate on the packing material and cause a reduction in the gas-liquid volumetric flow rates and, in extreme cases, a plugging of the tower. Therefore, alternative methods to using a packed bed tower are desired.
Recently, bubble columns have received much consideration as such an alternative to packed bed towers [(G. P. Narayan, M. H. Sharqawy, S. Lam, J. H. Lienhard V, and M. St. John. “Multistage bubble column humidifier”. Pat. Application Publication No. US 2014/0367871. Incorporated herein by reference in its entirety.] In a simplistic bubble column humidifier, hot water enters into the bubble column and accumulates until it reaches a certain level while air is concurrently injected into the column through a perforated plate or perforated pipe (sparger) located at the bottom of the column. This results in the formation of bubbles in the pool, or bath of accumulated hot water. In an alternative embodiment, bubble columns comprise upright columns provided with a multiplicity of spaced-apart stages each provided with a porous structure, sometimes referred to in the art as a bubble generator or bubble distributor.
Porous structures commonly used in these apparatus include those such as sieve plates and/or spargers comprising openings that can be described as pores, holes, or perforations. It is through these openings that a flowing stream of gas is passed so as to contact with a liquid stream being generally conveyed downwardly, and in cross-current flow, from stage to stage so as to maintain a substantially continuous phase. Such apparatus are commonly used to treat waste water or sea water in order to obtain pure or fresh water.
During a vapor-liquid contact process, such as a humidification process, if a pressurized gas is forced into a liquid through a fine network of openings in a porous structure, small diameter bubbles of the gas are formed in the liquid, resulting in a foam, or froth. This froth aids in the transfer of heat from the water to the gas. The smaller the bubbles, the higher the ratio of the carrier gas to the feed liquid so as to permit a greater efficiency in the mass and temperature transfer between a carrier gas and a feed liquid.
Previously, drilled porous plates, or spargers, possess openings that produce larger bubbles. As these bubbles do not as readily absorb a liquid component in vapor form, it is necessary to find those porous structures that can generate finer bubbles. Smaller bubbles increase the surface area to volume ratio and, therefore, speed up the mass and heat transfer reaction rates to save valuable processing time. Some porous structures have a ‘nozzle-like’ design, wherein the openings on the lower, or gas entrance side, are greater in diameter when compared to their respective openings on the upper, or gas exit side. This design provides a pressure advantage, and, if a lower internal pressure occurs, the finer pore openings (smaller diameter) are also used to prevent any liquid from penetrating back into the pores.
Although the finer pore size may favor the exchange of mass and heat between the cross flow streams, other factors including, but not limited to, the integrity of the porous structure and the amount of solids found in the liquid stream, should be considered when choosing a porous structure for a humidifier apparatus. It is noted that the smaller the porous openings, the greater risk of fouling of the openings by sediments, salts and other contaminants found in a feed liquid. Porous structures having too great of a total porous surface area often suffer from cracking, or breaking, with increased gas pressure stresses.
Further to mass and temperature transfer, this interchange by direct contact between a liquid, such as water, and a gas, such as air, is effected by causing the gas to bubble through a thermal layer, or layers of the liquid. The air bubbles, as they enter from a lower vantage point and pass through the water bath on each stage, provide for a larger surface area of contact between the gas and the heated liquid (to be cooled). The effect of such an arrangement is that the air streams or bubbles that form are continuously distorted and become subject to turbulence which is created as they pass through the much denser thermal layer or layers of the liquid bath. The result of this turbulence is that the interior of the air bubbles and air streams are brought to the interface of a cooler gas and a warmer liquid, and thereby heat transfer is promoted (achieved). Mass exchange occurs when the gas, now humidified, carries with it a percentage of steam, or water. The remaining liquid is now more concentrated than when it entered the humidifier apparatus, i.e. by having attained a higher concentration of a solute such as, but not limited to, minerals, salts, waste components and/or mixtures thereof. Systems that create air bubbles in water are found to be advantageous in mass and temperature transfer over those systems that created fine droplets of thin films of water.
Theoretically, to maximize the efficiency of mass and temperature transfer, all of a gas stream introduced to a multi-stage bubble column humidifier flows upwardly so as to pass through a series of liquid baths, each held in their respective chamber, until the gas reaches an uppermost section of an uppermost chamber and flows through—in its entirety-into the atmosphere, or into a capturing device held thereon. As stated, it is within this cross-current flow configuration that mass and heat transfer coefficients are maximized due to a diffusion of water (in the form of a vapor, or steam, component) into the bubbles of gas. Key to this transfer is the formation of a foam or ‘froth’ at the location where the air bubbles are dispersed throughout the water within the chamber(s) of the bubble column humidifier. The froth, by increasing the surface area for mass and heat transfer, also increases the rate of this transfer reaction.
Any impediments to the bubble formation process will directly impact the level of foam formation. For example, the central placement of a downcomer unit in previous vapor-liquid contact apparatus restricted valuable bubble distributor, or sparger, space. Also, seen in previous apparatus, the release of a liquid stream from a liquid bath held on one stage directly on top of the foam forming in an adjacent underlying chamber dampened the froth, or foam, formation.
However, although the presence of foam increases the rate of mass and heat transfer, the overall efficiency of the bubble column humidifier will decrease if the foam (bubbles) are allowed to return to a preceding, or prior, chamber. Therefore, an overproduction of foam is not preferred. In previous apparatus, high levels of foam resulted in the direct entry of bubbles into downcomer units so as to be transported to adjacent underlying chambers, thus reducing efficiency. Furthermore, if a high level of foam contacts the porous plate of the adjacent overlying chamber, this may result in a bursting of the bubbles and release of the vaporizable component, thus also reducing efficiency.
Therefore it is desirable to have a means for controlling an over-formation of foam, and for preventing the return of any bubble component to a previous chamber. As foam formation is mainly influenced by the air superficial velocity and the height of any water gates installed in a liquid-vapor contact apparatus, such as the bubble humidifier apparatus, a means for controlling this component is important. Having a watergate that is easily adjusted is an important factor.
A downcomer unit can be used as a watergate. Downcomer units may be contiguous with the humidifier vessel itself. If the downcomer unit is contiguous with the exterior shell of the bubble column humidifier, the watergate, or any part of the downcomer, may not be easily adjusted nor replaced in case of breakage. Additionally, the height of the water gate is not only important for foam formation, but also for liquid transfer concerns. Truncated water gate designs have several inherent limitations. For example, the water gate may not provide a sufficient length so as to cause the fluid in the chamber below to back up into the water gate. This will impede the flow of the liquid, and may even cause it to re-enter the chamber from which it originated. Also, any water gate releasing its contents at a location above the water bath increases the probability of sediments from the liquid, such as salts from seawater, accumulating on the porous structure. This may foul the openings of the porous structure, such as a sparger, so as to impede the flow of a gas, such as air, and reduce the efficiency of the apparatus. If the efficiency of a humidifier is compromised, the overall efficiency of a HDH system will also suffer, and thus limit the delivery of purified water.
Recent research has been directed to the development of an effective humidifier. For example, an approach as disclosed in U.S. Pat. No. 6,919,000 provides a reduction in the thermal resistance associated with incondensable gases by using a direct-contact condenser instead of a standard, indirect contact dehumidifier. This method increases the heat transfer rates in the condenser at the expense of energy efficiency, as the energy from the humid air entering the dehumidifier is not directly recovered to preheat the seawater. Thus, although the cost of the dehumidification device is reduced, the energy costs associated with this method actually increase.
Another alternative approach as disclosed in U.S. Pat. No. 4,045,190 provides a method for regulating the flow of liquid through mass transfer columns using throttle valves responsive to a gas pressure drop or a height of a liquid. However, some bubbles may leave with the water stream; subsequently reducing the heat recovery in the humidification process.
Further to this approach, U.S. Pat. Nos. 6,250,611 and 6,575,438 both describe vapor-liquid contact apparatus for use in carrying out chemical processing which use downcomer(s) fixed with a sparger plate. However, in these two references, it is not possible to adjust the height of water level, and, most significantly, numerous bubbles may leave with the water stream.
A simple, low cost vapor-liquid contact apparatus, such as a multi-stage bubble column humidifier apparatus, providing a means to limit the loss, or return, of a vapor component so as to achieve an increased efficiency in mass and/or heat transfer, is greatly desired.